This invention pertains generally to the field of power conversion and, more particularly, to switching power supplies with feedback control.
Compact and efficient power supplies are an increasing concern to users and manufacturers of electronics. Switching power supplies with pulse width modulated (xe2x80x9cPWMxe2x80x9d) controllers offer both compactness and efficiency in a number of different topologies. Boost and buck switching power supply topologies are efficient, but do not isolate the power input from the power output. Other topologies, such as the flyback, do isolate the power input from the power output by using a transformer. In such topologies, feedback from the secondary (power output) side of the transformer is needed to adjust the pulse width modulation duty cycle of the power switch. PWM control for a switching power supply may be provided from a single integrated circuit chip or package having some number of external connection pins or terminals. As with many other types of integrated circuit chips or packages, limiting the number of external connection terminals of a power supply package can be advantageous.
For example, U.S. Pat. No. 5,313,381 to Balakrishnan (the xe2x80x9c""381 patentxe2x80x9d), which is fully incorporated by reference, discloses a three-terminal switching power supply control chip for use with a flyback converter. FIG. 1 illustrates a flyback converter 20 according to the ""381 patent. The converter 20 employs a three-pin control chip 22 to supply current from a rectified DC source (Vbb) 28 across an isolating transformer 24 to supply power for a load 26. The power supply chip 22 includes a first terminal 30 coupled to a primary winding 32 of the transformer 24, a second (xe2x80x9cgroundxe2x80x9d) terminal 36 coupled to a primary side ground reference, and a third terminal 40 for accepting a combined feedback control signal (IFB) and a bias supply voltage (Vcc) to operate the control chip 22.
Within the power supply chip 22, the first terminal 30 is alternately coupled to the ground terminal 36 by a power transistor switch 42. PWM control circuitry 44 drives the power switch 42 at a variable duty cycle. When the power switch 42 is ON, current flows through the primary winding 32 and energy is stored in the magnetic core 45 of the transformer 24. When the switch 42 is OFF, a secondary diode 46 is forward biased and the stored energy in the transformer core 45 is released through a secondary winding 48 to a filter/storage capacitor 47 and the load 26. After the transformer 24 is reset, the ON/OFF cycle is repeated.
An error amplifier 50 compares the output voltage Vout across the load 26 with a reference voltage to generate the feedback control signal IFB. The bias supply voltage Vcc is supplied from an auxiliary secondary winding 52 of the transformer 24. The bias supply voltage Vcc is modulated with the feedback control signal IFB in an opto-isolator 54 to create the combined bias voltage, feedback signal Vcc/IFB. A feedback extraction circuit (not shown) in the chip 22 separates the feedback signal IFB from the bias voltage Vcc by sensing the excess current flowing through a shunt regulator. The extracted feedback signal IFB is used to control the output of the PWM circuitry 44 to constantly adjust the duty cycle of the power switch 42 so as to transfer greater or lesser current to the secondary.
Notably, to properly compensate the PWM controller based on feedback from the secondary requires extra components and often involves expensive re-design, depending upon the particular application. Yet, prior art isolated power supplies that used feedback only from the primary side of the transformer do not account for power losses encountered on the secondary side of the transformer. See, e.g., U.S. Pat. No. 5,982,644, (the ""644 patent), which discloses a pulse-width-modulated boost converter coupled to a high voltage converter, which in turn is coupled to the primary side of a transformer. The modulation of the boost converter is adjusted according to an amplified error signal representing the difference between the boost converter""s output voltage and the voltage from a current sensing circuit sensing the current through the primary winding. This error signal has no way of sensing and accounting for the losses on the secondary side of the transformer. Thus, the power supply disclosed in the ""644 patent employs a linear regulator on the secondary side of the transformer to maintain a constant voltage over the load. Although this power supply avoids the use of feedback from the secondary side of the transformer, it introduces the expense and loss associated with installing an additional regulator at the load.
Thus, it would be desirable to provide minimal terminal power supply packages for controlling more complex power converter topologies, including packages that isolate the input and output through a transformer without requiring feedback from the secondary side of the transformer, thereby easing design and reducing the component count.
In accordance with a first aspect of the invention, a three-terminal power supply package is provided for controlling delivery of power from a source to a load. In a preferred embodiment, the three-terminal package has a first terminal for coupling to a primary winding of a transformer, a second terminal for coupling to a ground reference, a third terminal for coupling to a source of operating power, and an internal power switch. The internal power switch has an input coupled to the first terminal, an output coupled to the second terminal, and an activation gate. The control package further includes pulse train control circuitry coupled to the power switch activation gate and responsive to an error signal for driving the power switch. The error signal is derived from an internally generated compensation signal corresponding to an expected voltage loss between the source and the load. The control package may be used to control a number of single switch, transformer coupled power converter topologies, including (by way of example only) flyback and combined forward-flyback converters. Depending on design considerations, the internal power switch and pulse train control circuitry may be formed as part of a single integrated circuit.
In accordance with another aspect of the invention, a four-terminal power supply package is provided for controlling delivery of power from a source to a load. In a preferred embodiment, the control package has a first terminal for coupling to a primary winding of a transformer, a second terminal for coupling to a ground reference, a third terminal for coupling to a source of operating power, and a fourth terminal for coupling to an external compensation circuit for generating a compensation signal corresponding to an expected voltage loss between the source and the load. The package includes an internal power switch having an input coupled to the first terminal, an output coupled to the second terminal, and an activation gate. The control package further includes pulse train control circuitry coupled to the internal power switch activation gate and responsive an error signal for driving the power switch, the error signal derived from the compensation signal received on the fourth terminal. The control package may be used to control a number of single switch, transformer coupled power converter topologies, including (by way of example only) flyback and combined forward-flyback converters. Depending on design considerations, the internal power switch and pulse train control circuitry may be formed as part of a single integrated circuit.
In accordance with yet another aspect of the invention, a four-terminal power supply package is provided for controlling delivery of power from a source to a load. In a preferred embodiment, the control package has a first terminal for coupling to a primary winding of a transformer, a second terminal for coupling to an activation gate of an external power switch, a third terminal for coupling to a ground reference, and a fourth terminal for coupling to a source of operating power. The package includes an internal power switch having an input coupled to the first terminal, an output coupled to the third terminal, and an activation gate. The control package further includes pulse train control circuitry coupled to both the internal power switch activation gate and the second terminal (for driving the externally power switch). The pulse train control circuitry is responsive to an error signal for driving the respective power internal and external power switches. The error signal is derived from an internally generated compensation signal corresponding to an expected voltage loss between the source and the load. The control package may be used to control a number of dual switch, transformer coupled power converter topologies, including (by way of example only) push-pull, half bridge and interleaved flyback. Depending on design considerations, the internal power switch and pulse train control circuitry may be formed as part of a single integrated circuit.
In accordance with a further aspect of the invention, a five-terminal power supply package is provided for controlling delivery of power from a source to a load. In a preferred embodiment, the control package has a first terminal for coupling to a primary winding of a transformer, a second terminal for coupling to an activation gate of an external power switch, and third terminal for coupling to a ground reference, a fourth terminal for coupling to a source of operating power, and a fifth terminal for coupling to an external compensation circuit for generating a compensation signal corresponding to an expected voltage loss between the source and the load. The package includes an internal power switch having an input coupled to the first terminal, an output coupled to the third terminal, and an activation gate. The control package further includes pulse train control circuitry coupled to both the internal power switch activation gate and the second terminal (for driving the externally power switch). The pulse train control circuitry is responsive to an error signal for driving the respective power internal and external power switches, the error signal derived from the compensation signal received on the fifth terminal. The control package may be used to control a number of dual switch, transformer coupled power converter topologies, including (by way of example only) push-pull, half bridge and interleaved flyback. Depending on design considerations, the internal power switch and pulse train control circuitry may be formed as part of a single integrated circuit.
In accordance with a still further aspect of the invention, a four-terminal power supply package is provided for controlling delivery of power from a source to a load. In a preferred embodiment, the control package has a first terminal for coupling to a first primary winding of a transformer, a second terminal for coupling to a second primary winding of the transformer, a third terminal for coupling to a ground reference, and a fourth terminal for coupling to a source of operating power. The control package includes both first and second internal power switches, the first power switch having an input coupled to the first terminal, an output coupled to the third terminal, and an activation gate, and the second power switch having an input coupled to the second terminal, an output coupled to the third terminal, and an activation gate. Pulse train control circuitry is coupled to the first and second power switch activation gates, the pulse train control circuitry responsive to an error signal for driving the respective first and second power switches. The error is derived from an internally generated compensation signal corresponding to an expected voltage loss between the source and the load. In one preferred implementation, the control package is employed to control a push-pull power converter. Depending on design considerations, the first power switch, second power switch, pulse train control circuitry, or some combination thereof, may be provided in a single integrated circuit.
In accordance with still another aspect of the invention, a five-terminal power supply package is provided for controlling delivery of power from a source to a load. In a preferred embodiment, the control package has a first terminal for coupling to a first primary winding of a transformer, a second terminal for coupling to a second primary winding of the transformer, a third terminal for coupling to a ground reference, a fourth terminal for coupling to a source of operating power, and a fifth terminal for coupling to an external compensation circuit for generating a compensation signal corresponding to an expected voltage loss between the source and the load. The control package includes both first and second internal power switches, the first power switch having an input coupled to the first terminal, an output coupled to the third terminal, and an activation gate, and the second power switch having an input coupled to the second terminal, an output coupled to the third terminal, and an activation gate. Pulse train control circuitry is coupled to the first and second power switch activation gates, the pulse train control circuitry responsive to an error signal for driving the respective first and second power switches. The error signal is derived from the compensation signal received on the fifth terminal. In one preferred implementation, the control package is employed to control a push-pull power converter. Depending on design considerations, the first power switch, second power switch, pulse train control circuitry, or some combination thereof, may be provided in a single integrated circuit.
Other objects and features of the present inventions will become apparent hereinafter.